To prevent compressor surge, gas turbine engines employ surge control systems. These control systems use corrected discharge flow W.sub.dc, defined by equation (1), from the compressor to sense the proximity of surge. EQU W.sub.dc =(W.sub.d .times..sqroot.T.sub.a /T.sub.s )/(P.sub.d /P.sub.s)(1)
Where W.sub.d is compressor discharge air flow, T.sub.a is discharge temperature, T.sub.s is standard day temperature, P.sub.d is total discharge pressure, and P.sub.s is standard day pressure.
Shown in FIG. 1, is a conventional pitot-static flow sensor used by some surge control systems to determine corrected discharge flow. Compressor discharge air flows through a conduit 2 and then is directed to the aircraft and/or overboard. The sensor includes a total pressure transducer 4 for measuring the total pressure in the conduit and a delta pressure transducer 6 for measuring the difference between total and static pressure in conduit 2. The sensor also has a variable volume chamber 8, a filter 10, and a directional control orifice 12. An electronic control box, ECB 14, containing a microprocessor receives the signals from the transducers, calculates (P.sub.d -P.sub.static)/P.sub.d and uses this calculated value to enter a look-up table or algorithm to determine W.sub.dc. If W.sub.dc is above a specified value indicative of a possible surge the ECB will command the surge control valve 16 to open. Disadvantages to this type of control system are the complexity of the pitot-static flow sensor and the dependence on look-up tables or algorithms. As the engine ages, these tables and algorithms become less and less representative of actual engine conditions, resulting in significant differences between the W.sub.dc used in the ECB and the actual W.sub.dc in the engine.
FIG. 2 shows a much simpler arrangement that only uses the total pressure transducer and a hot wire anemometer. The pressure transducer provides a pressure signal, and the hot wire anemometer provides both a signal proportional to discharge flow, and a temperature signal. The ECB receives these signals and then calculates the corrected discharge flow, thus eliminating look-up tables. A disadvantage to this system is the slow dynamic response of the hot wire anemometer to an abrupt or step change in flow. Because surge can occur very rapidly and cause catastrophic damage to the engine, the slow dynamic response of hot wire anemometers has limited their effectiveness in surge control systems.
Accordingly, there is a need for a surge control system in which the dynamic response of a hot wire anemometer is canceled from the measurement of compressor discharge flow without adversely affecting the accuracy of the measurement.